The present invention is generally directed to the field of electrically resistive materials. In particular, the present invention is directed to electrically resistive materials that can be deposited as thin films and patterned to form discrete resistors that may be embedded in printed circuit boards.
U.S. Pat. No. 6,210,592 describes thin film resistive material that may be patterned to form discrete resistive elements (resistors) that may be embedded in printed circuit boards. The thin films are formed predominantly of platinum. To increase the resistance, various levels, typically from 0.1 to 20 wt %, particularly from 0.5 to 5 wt %, of total material is a metal oxide and/or a metalloid oxide. The most thoroughly discussed oxide in this application is silica, although others, such as alumina and ceria, are also noted. The thin films described in this application are preferably formed by combustion chemical vapor deposition (“CCVD”) as described in U.S. Pat. No. 6,562,021 or controlled atmosphere chemical vapor deposition (“CACVD”) as described in European Patent Application EP 976 847 A2.
It may be appreciated that devices that utilize printed circuit boards (“PCBs”) as components may be exposed to temperature extremes, and many industry specifications require performance over a temperature range of from −50° C. to 150° C. Over such temperature ranges the electrical performance of the printed circuit board must remain relatively constant for the device to function properly. A problem that has been noted with platinum based thin film resistors, such as those described in above-described U.S. Pat. No. 6,210,592, is that the thermal coefficient of resistivity (“TCR”) tends to be greater than is required for many PCB applications. For consistent electrical performance over broad temperature range, it is desired that the TCR of a material be as low as possible.
Accordingly, it is a general object of the present invention to provide electrically resistive materials with low TCRs, particularly such a materials that may be deposited as a thin film, patterned to form discrete resistors, and embedded in printed circuit boards.